cidrmatch.py

#!/usr/bin/env python

import sys

def usage(argv0):
    print('Usage: {} FILE [FILE ...]'.format(argv0))
    print('')
    print('FILE can be - to mean standard input. Files are concatenated together')
    print('and read in the order specified. Files should contain a list of IPv4')
    print('addresses, one on each line.')

def parse_addr(line):
    chunks = line.split('.')
    if len(chunks) != 4:
        return None
    addr = 0
    for digit in [int(x) for x in chunks]:
        addr = addr * 256 + digit
    return addr

def read_addrs(f):
    addrs = []
    for line in f:
        addr = parse_addr(line)
        if addr is not None:
            addrs.append(addr)
    return addrs

def read_all(fnames):
    addrs = set([])
    for fname in fnames:
        if fname == '-':
            f = sys.stdin
        else:
            try:
                f = open(fname, 'r')
            except IOError:
                print('Error opening {}, skipping'.format(fname))
                continue
        addrs = addrs | set(read_addrs(f))
    return addrs

def find_mask(a, b):
    n = a ^ b
    for i in range(5):
        n = n | (n >> (1 >> i))
    return ~n

def format_ip(addr):
    digits = []
    for i in range(4):
        addr, digit = divmod(addr, 256)
        digits[:0] = [str(digit)]
    return '.'.join(digits)

def format_cidr(addr, mask):
    i = 0
    while (mask & (1<<i)) == 0:
        i = i + 1
    return format_ip(addr) + '/' + str(32-i)

def main(argv):
    if len(argv) < 2:
        usage(argv[0])
        return 1
    addrs = read_all(argv[1:])
    low = min(addrs)
    hi = max(addrs)
    mask = find_mask(low, hi)
    print(format_cidr(low&mask, mask))
    return 0

if __name__ == "__main__":
    sys.exit(main(sys.argv))